Kind of like those universal language codes we see around electrical power installations:
The placenta is a defining feature of being a mammal, and its formation is one of the first steps in mammalian development. The embryo begins to make its placenta without direct guidance from its mother — rather, it follows a set of molecularly encoded, do-it-yourself assembly instructions. Whether these instructions are universal or unique to each species of mammal is a long-standing mystery. Writing in Nature, Gerri et al.1 report a remarkable similarity in how mouse, cow and human embryos make their placentas.…
The knowledge gleaned from embryos thus guides our understanding of how to optimize protocols to manipulate the identity and function of stem cells, as well as bringing us closer to understanding the universal assembly instructions for mammalian embryogenesis.Jennifer L. Watts & Amy Ralston, “Universal assembly instructions for the placenta” at Nature
So was there a single “ancestral mammal” or is it a common design (convergent evolution)? Either is quite plausible today.